In conventional computer data storage systems having a rotating storage medium, data is stored in a series of concentric or spiral tracks across the surface of a disk. Each track includes a number of sectors. The storage medium can comprise, for example, a disk having a surface on which a magnetic material is deposited, such as conventional magnetic disks or magneto-optical disks. The data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits are read from the disk surface by a magnetic head transducer suspended over the disk surface that can detect the variations in magnetic orientation as the disk rotates relative to the magnetic head at thousands of revolutions per minute and generate a fluctuating data signal.
Conventionally, the magnetic head is mounted on a disk arm or carriage that is incorporated in a servo system that moves the head, via an actuator, radially in a "seek" or "access" function; i.e., the servo system moves the head to a selected track from a previous track. When the head reaches the desired track, the servo system commences a "track following" function in which it accurately positions the head over the centerline of the selected track and maintains it in that position so that the head can record a series of data bits or, alternatively, retrieve a series of bits from the track as the disk rotates under the head. Thus, the disk drive servo control system controls movement of the arm across the surface of the disk to move the head from track to track and, once over a selected track, to maintain the magnetic head within a given tolerance of distance over the centerline of the desired data track during read or write operations. In a track seek operation, the magnetic head is moved over the disk to a desired one of the tracks. To accurately position the magnetic head at the desired track, it is necessary to determine the track number beneath the head as the disk rotates and the head is moved across the disk.
One such system is a digital sector servo control system that is used to maintain the magnetic read/write head precisely over a desired track during a read or write operation. Such a servo system requires that servo information be prerecorded on the disk file. Servo information can be prerecorded on either a dedicated servo surface or on servo sectors located on each disk surface or on a combination of both.
During both seeking and track following operations, the prerecorded servo information is sensed by the head and demodulated to generate a digital gray code and a position error signal (PES). The digital gray code includes track identification information and the PES indicates the position of the head away from the centerline of a track (i.e., an offset from the center of the track). The digital gray code and the PES are combined together to generate a measured position signal. The measured position signal is then used in a servo feedback loop to generate a control signal to move the head back to the centerline of the target track.
In other words, each disk stores servo information in different sectors of the disk required for positional control. The sector servo method reproduces servo information written on the disk to determine the track number and the exact position of the head relative to the center of the track. A description of a general digital disk file servo control system is given by U.S. Pat. No. 4,679,103 granted to Michael I. Workman and titled "Digital Servo Control System For a Data Recording Disk File".
Servo sectors or bursts are angularly spaced pie-piece-shaped sectors which are interspersed among the data sectors on the data disks. The servo signals may be embedded in the data recorded in servo fields at the beginnings of the data track sectors, for example. Alternatively, the servo signals may be recorded on a disk surface dedicated to servo signals. All of these mechanisms for servo control information are well known to those of ordinary skill in the art. The prerecorded servo information is normally written as servo sectors or bursts to the disks by a servowriter at the factory, before the disks are shipped to users. The prerecorded servo information, including servo bursts, is read from the disks, demodulated, and processed by the servo control system. The results are then applied to the input of the servo electronics which control the current to the actuator such as a voice coil motor (VCM) and thus the radial position of the head over the disk surface.
As described in further detail below, disk drives accept removable disk cartridges that contain a flexible magnetic storage medium upon which information can be written and read. The disk-shaped storage medium is mounted on a hub that rotates freely within the cartridge. A spindle motor within the drive engages the cartridge hub when the cartridge is inserted into the drive, in order to rotate the storage medium at relatively high speeds. The spindle motor does this by first moving from an unloaded position to a loaded position when a disk cartridge is inserted into the disk drive. In the loaded position, the spindle of the disk drive motor contacts the hub of the disk cartridge. The spindle can then be rotated in order to rotate the hub and the storage medium of the disk cartridge. A shutter on the front edge of the cartridge is moved to the side during insertion into the drive, thereby exposing an opening through which the read/write heads of the drive move to access the recording surfaces of the rotating storage medium. The shutter covers the head access opening when the cartridge is outside of the drive, to prevent dust and other contaminants from entering the cartridge and settling on the recording surfaces of the storage medium.
There are several reasons for the position of a read/write head to be in error, or off track, during a track following operation. One of the major components of head position error is called repeatable run out (RRO) at the disk rotating frequency which is an error caused by an unbalanced spindle or by a non-ideal bearing. The Workman U.S. Pat. No. 4,679,103 does not specifically deal with this problem. The servo control system disclosed in the Workman U.S. Pat. No. 4,679,103 does not have sufficient gain at the run out frequency to fully correct for the RRO error.
Particularly where a disk is removable from the drive, the primary source of off track in a removable cartridge disk drive is the misalignment between the center of motor spindle and the hole in the cartridge hub. When there is a misalignment, the center of the circular track on the disk does not coincide with the center of the motor rotation. As a result, the linear actuator has to move towards the center of the disk during half of the revolution and away during the other half in order to follow the track on the disk. This results in a 1 f runout frequency signature. It is typically desirable to reduce the runout.
One of the forces the actuator has to overcome during track following is the friction force between the actuator and the center rod on which the actuator is sliding on. This friction force is against the motion of the actuator. Therefore, every time the actuator changes its direction of motion, the net force change due to friction is twice the amplitude of the friction force. Because of the sudden change in the forces on the actuator and the time it takes for the servo system to learn of this change, the recording head will deviate from the ideal track starting at the direction turn-around for some time. This particular off track is termed friction peak.
Regularly, there are only two friction peaks per every revolution due to two turn-arounds per revolution. Because the friction peaks occur at the fixed angular position per every insertion, the servo systela can be programmed to learn this phenomenon and fix the off track after short learning period. However, complication arises if there exists some significant runout that repeats twice per revolution (i.e., 2f runout). Depending on the ratio of the 1f and 2f runout amplitude, it could occur that there are four directional turn-arounds per revolution giving rises to four friction perk per revolution. For a servo system to handle both the two friction peak case and the four friction peak cases, the programming becomes too complicated.
Thus, oftentimes, there is a transient 2f problem in the PES measurement. This phenomenon is due to changes in the ratio of the amplitudes of the 1f and 2f radial runout which leads to friction in the electromechanical servo system. This friction causes off track error, particularly at the time of a reversal in the direction of motion of an actuator which moves the magnetic head. During some disk insertions, as the tracks are aligned with the motor spindle center line, if runout amplitude becomes reduced and 2f radial runout becomes significant. When sufficiently large 2f runout is present, actuator mechanics experience additional directional reversals over one revolution (e.g., four directional reversals instead of the usual two) which results in additional friction bumps per revolution (e.g., two additional friction bumps). These friction bumps emulate a 2f runout signature and contribute to an apparent 2f radial runout. In other words, the apparent 2f radial runout=the actual 2f radial runout+the signal due to the friction. Thus, the magnitude of the resultant apparent 2f PES amplitude becomes largely independent of the actual 2f radial runout and dependent on the frictional characteristics of the actuator system. Although the actual 2f radial runout remains substantially constant, the apparent 2f radial runout is increased. The apparent 2f radial runout affects the PES (i.e., alters the PES value from its true value to a perceived value), thus causing the head to become misaligned over the disk surface.
Although the art of read/write head positioning is well developed, there remain some problems inherent in this technology, particularly induced friction. Therefore, a need exists for a servo method and system that overcomes the drawbacks of the prior art